Any radioshack store in the UK sell batteries quite cheap, but they only sell cells and not packs, if that makes sense? If they havn't got the ones you want they will order them. Overlander are good too, well their delivery is, i ordered 6 props on sunday morning and they came this morning(Wednesday).
Matthew

For 'standard' packs I tend to shop around (Al's Hobbies, West London models etc.) but Overlander are hard to beat and I've always had good turnaround on orders. Their 'own brand' Nimh packs are excellent value too.